Ataxia-telangiectasia (A-T) is a profoundly destructive childhood disorder resulting from mutation of a gene encoding a member of the PI3 kinase family. The gene, named ataxia- telangiectasia mutated (ATM) is a recognized component of the response mechanism to DNA double strand breaks. Yet, the consequences of ATM mutation include dramatic neurological deficiencies, and while some features of A-T can be explained by a faulty response to DNA damage, it is hard to fully account for the neurological aspects of the disease on this basis. The core hypothesis of this application is that there is a nervous system-specific form of ATM that is cytoplasmic. In this location, it serves as a serine-threonine kinase but equally as a chaperone protein that nucleates a complex of proteins that includes vesicle proteins such as synapsin-I and VAMP2. We propose that it is the failure of the functions associated with cytoplasmic ATM that leads to many of the neurological symptoms of A-T. We will explore this hypothesis in a two-part strategy. Our first aim is to expand our understanding of the role of ATM as a nucleocytoplasmic protein in neurons, and to uncover the unique functions that are enabled by its dual localization. We will explore how cytoplasmic ATM contributes to neuronal function on its own, and how it coordinates its action with its counterpart in the nucleus. Our outcome measures will include dendritic morphology, synaptic physiology, cell cycle regulation and cell survival. The second part of our strategy will be to develop strong pre-clinical platforms, both in vivo and in vitro, to test several agents currently under study in the clinic. We provide preliminary data showing that the well-described diversity of phenotype in human A-T is mimicked successfully in the mouse, provided that the correct set of alleles is chosen for analysis. We will use this newly expanded view of the mouse models to test the involvement of ATM in the neuronal response to stresses such as excitotoxicity, DNA damage and oxidative stress. Using the same outcome measures as above we will specifically test wild type and Atm- deficient cells for their response to stress and compare anti-oxidant and anti-inflammatory strategies for the potency as protective agents. Successful leads will be followed by in vivo trials comparing wild type with two different Atm mutations and their compound heterozygote. In a final series of pilot experiments we will explore whether ATM malfunction plays a role in other neurodegenerative disease. In the aggregate these studies offer a new look at the neurological deficits associated with A-T and, through the development of in vitro and in vivo model systems, brings fresh momentum to the discovery of treatments for this devastating condition.